Paint coatings are surface protective coatings applied to substrates and dried to form continuous films for decorative purposes as well as to protect the substrate. Consumer paint coatings are air-drying aqueous coatings applied primarily to architectural interior or exterior surfaces, where the coatings are sufficiently fluid to flow out, form a continuous paint film, and dry at ambient temperatures to protect the substrate surface. A paint coating ordinarily comprises an organic polymeric binder, pigments, and various paint additives. In dried paint films, the polymeric binder functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness, although some paint coatings contain little or no opacifying pigments and are described as clear coatings. The manufacture of paint coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in a dispersant medium, and thinning to commercial standards.
Latex paints for the consumer market ordinarily are based on polymeric binders prepared by emulsion polymerization of ethylenic monomers. A typical consumer latex paint binder contains a vinyl acetate copolymer consisting of polymerized vinyl acetate (80%) and butyl acrylate (20%), although many paint binders comprise numerous combinations of copolymerized vinyl and/or acrylic ethylenically unsaturated monomers. The hardness of the latex polymer must be balanced to permit drying and film formation at low application temperatures, which requires soft polymer units, while at the same time the polymer must be hard enough in the final dried film to provide film integrity resistance properties, which requires hard polymer units. This conventionally is accomplished by designing a latex polymer with a moderately elevated Tg (glass transition temperature) having a Tg above the ambient dry temperature, but then lowering the Tg temporarily with a volatile coalescing solvent. Coalescing solvents function to externally and temporarily plasticize the latex polymer for time sufficient to develop film formation, but then the coalescing agents diffuse out of the film by the volatilization of the coalescent. If a lower Tg binder copolymer is used without a volatile coalescing solvent, higher levels of soft comonomer are required, but the final dried film of lower Tg polymers would be undesirably soft, excessively tacky, readily stain, and easily pick up dirt.
A significant source of residual odor in latex consumer paints is directly due to the volatile coalescing solvent. Coalescing solvents are typically linear (or slightly branched) glycol ethers and esters of 7 to 12 carbon atoms in length. One typical coalescing solvent ordinarily contained in commercial latex paints is 2,2,4-trimethylpentanediol monoisobutyrate. The odor associated with the gradual volatilization of this solvent is considered objectionable by consumers. Quite often the odor lingers for days after the paint is applied and dried. All useful coalescing solvents are volatile and have similar objectionable characteristics.
Commonly assigned U.S. Pat. Nos. 5,326,808 and 5,422,392 and 5,470,946 teach odor free air dry latex paints where the polymeric binder is modified with a non-volatile oligomeric polyurethane or a polyester having a Tg below −20 degrees C. to produce an air dry paint free of objectionable odors and volatile coalescing solvents. Similarly, commonly assigned U.S. Pat. No. 5,346,948 teaches a low molecular weigh chlorinated hydrocarbon as a non-volatile modifier to avoid the need for volatile coalescing solvents.
In the field of thermoplastic injection molding, U.S. Pat. No. 5,728,761 discloses high molecular weight polylactic acid molding compounds having molecular weights between 50,000 and 300,000 modified with low molecular weight lactic acid derivatives used as placticizers in the molded product. The low molecular weight lactic acid derivatives are linear esters or polyesters produced by esterifying the carboxyl groups of the lactic acid with alkyl or aryl groups along with alkylation of the hydroxyl group with alkyl, aryl, acyl, or silyl groups.
It now has been found that low molecular weight oligomers of poly(hydroxy acid) such as poly(lactic acid) can be used as non-volatile film forming modifiers in latex aqueous emulsion paints to assist room temperature film forming without the use of objectionable volatile coalescing solvents. Particularly useful poly(hydroxy acid) oligomers are alkyl ester terminated and further contain hydrophilic terminal hydroxyl groups to facilitate direct mixing of the oligomer into the aqueous emulsion latex paints. By using non-volatile, external softening oligomers in accordance with this invention, the relationship between Tg (or MFT) and film forming temperature of the polymeric binder in the final paint can be changed such that both film formation and tack-free character are simultaneously obtained without the need for a volatile coalescing solvent.
Preferred poly(hydroxy acid) oligomers are poly(lactic acid) hereafter “PLA” and poly(glycolic acid) hereafter “GLA”. Lactic acid is a naturally occurring organic hydroxy acid comprising an alpha hydroxy propionic acid containing both carboxyl and hydroxy groups, which copolymerize by self esterification to form linear polylactic acid. Glycolic acid likewise is a naturally occurring organic hydroxy acid comprising hydroxyacetic acid. Due to combined hydrophobic and hydrophilic characteristics, the PLA and GLA oligomers are compatible with typical aqueous emulsion vinyl acetate polymers copolymers and acrylic polymers copolymers commonly used as binders in latex paints. The PLA and GLA oligomers of this invention can be retained permanently and will not volatilize out of the final paint film. Hence, applied paint films will not generate an odor while drying nor emit a residual odor from the dried paint film or otherwise emit VOC's. The softening oligomeric modifiers of this invention externally modify the vinyl or acrylic binder matrix polymers and do not coreact with the matrix polymer. The oligomers are particularly useful as non-volatile coalescing aids in latex paints. The softening oligomeric modifiers of this invention appear to soften the binder polymers in aqueous liquid form but remain in the film and reinforce the matrix polymeric binder during drying to provide low temperature film formation and tack-free films not prone to soiling at a given hardness and/or flexibility.
A problem with polyester or polyurethane oligomers used to modify latex paints, such as shown in U.S. Pat. No. 5,326,808, is that conventional polyester oligomers comprising glycols esterified with dicarboxylic acids can not be directly mixed with aqueous emulsion latexes. Such polyester oligomers need to be dissolved in the monomer, emulsified, and then the monomers copolymerized in situ with the conventional polyester oligomer, or alternatively, the oligomer must be pre-emulsified in water before mixing with latex polymer. In contrast, the oligomers of this invention can be separately preformed and added directly to the latex or resulting paint as an aqueous emulsion. Another deficiency with conventional glycol polyester in modified latex paints is that dried paint films have fair scrub resistance at best. In contrast, the oligomer modified paints of this invention provide dry paint films exhibiting excellent scrub resistance. Another deficiency is that conventional glycol dicarboxylic acid polyesters tend to hydrolyze and can generate toxic materials.
Accordingly, a major advantage of this invention pertains to dried paint films exhibiting superior toughness obtained through the use of a film forming matrix polymer balanced with the poly(hydroxy acid) oligomeric modifier to obtain liquid phase softening through the addition of softening modifier while retaining the desired dried film hardness. Volatile coalescing agents can be eliminated and binder volume can be increased by using a permanently retained non-volatile softening oligomer instead of a volatile coalescent. The resulting dried paint films exhibit a superior balance of hardness and flexibility while maintaining long term flexibility. As an added benefit, the oligomers are derived from renewable sources such as corn syrup, beet sugar, or agricultural waste by fermentation, and are based on bio-renewable materials very safe for human contact. These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.